As the world's population increases, the demand for water also increases. Indeed, in some parts of the world where the local population is growing at a much higher rate than average, the availability of safe drinking water is lower than average. Some of this situation can be attributed to geography, whether from an arid climate or simply the lack of fresh surface water suitable for drinking. Additionally, many wellheads are running dry due to the lowering of underground aquifers, resulting in new wells being drilled to deeper depths, in an attempt to find water. In many cases, high costs prohibit these operations. Further, in many locales where water is very scarce, the population is unable to purchase water for consumption due to their low income levels and the fact that municipally treated water is unavailable. Examples of such settings may include rural villages in under-developed countries, emergency relief sites following natural disasters, or camp settings, to name a few.
Modern municipal water treatment systems, where available, are equipped to treat and distribute water for human consumption. In many cases, this treatment involves coagulation, flocculation and sedimentation of particulate matter. Additional filtering of the water may also be conducted, as well as treatment with chlorine. Due to the nature of a municipal system, the treated water may not be consumed immediately, and the chlorine remains in the water until it is dispensed.
When water is treated in a home beyond a municipal system (if one is available) the system is commonly referred to as a point-of-use (POU) system. These home POU systems use a variety of processes to treat water, such as: screening, reverse osmosis, carbon adsorption, deionization, softening, boiling, distillation and UV irradiation. Many POU systems are intended for homes with reliable access to supply water at relatively high pressure (>20 psi). Additionally, these homes generally have access to electricity or other energy sources to operate pumps to pressurize water and to run electronic devices generally found in some POU systems. Most of these systems require potable water to be supplied at the inlet.
As a result, there is a need for a home POU system for those who lack access to potable municipal water and who may not have access to electric power or other energy sources. People without a municipal water system seeking water may bring a container to a source, such as a well, stream, or lake, and obtain water directly. This water is either stored in containers or collected in a larger vessel for future use. If available, treatment is typically limited to simple pour-through screening or sand filtration. The bio-sand filters commonly used in residential and small village settings tend to be large and heavy. Some contain as much as 100 pounds of sand and gravel. These bio-sand filters are marginally effective at trapping microbes and particles and they typically produce water that is visibly clearer and relatively free of disease causing microbes. However, these systems tend to act as a chromatographic column meaning particles are trapped at various rates as the water moves down the column. The result is that eventually fine particles (thought to be trapped in the sand) begin to break through into the effluent water.
In some cases users allow the water to sit for a period of time, to allow for particles in the water to settle to the bottom of the container—sedimentation. In other cases, chemicals are added to the water to increase the speed of this process. These chemicals are sometimes called flocculation agents, such as alum or poly aluminum chloride. However, the water, even after this treatment, still needs to be disinfected, destroying microbes. Boiling may be the simplest treatment to destroy microbes or microorganisms, but requires an energy source. Another option is a bio-sand filtration unit. An exemplary bio-sand filtration unit (200) is shown in FIG. 2 and a flowchart illustrating a bio-sand filtration unit is shown in FIG. 1. These are less effective than boiling water, with the possibility of the resulting water still containing harmful microbes. Chlorine may be added to the water, for example, using the system shown in FIG. 3. But, the unfamiliarity of the taste chlorine adds to the water, combined with the unit volume required to achieve an effective treatment, leads many users to discontinue using the chlorinated water due to the offensive taste. As a result, these users often return to using untreated water, which perpetuates the cycle of illness and poor health.
In a publication entitled “Four Layer System” Dr. David H. Manz describes the effectiveness of bio-sand filters in terms of the maximum recommended face velocity of water through the exposed face area of the filter. He recommends that 600 liters per hour or flow per meter squared of exposed filter surface area as the maximum face flow rate per filter face area. This translates (through unit reduction) to a face velocity of 1 cm per minute.
                                             V            ⁢                                                  ⁢            max                    =                    ⁢                      maximum            ⁢                                                  ⁢            recommended            ⁢                                                  ⁢            face            ⁢                                                  ⁢            velocity                                                                    V            ⁢                                                  ⁢            max                    =                    ⁢                                    600              ⁢                                                          ⁢              1              ⁢                              /                            ⁢                              hr                /                                  m                  ^                  2                                                      =                          10              ⁢                                                          ⁢              1              ⁢                              /                            ⁢                              min                /                                  m                  ^                  2                                                                                                              =                        ⁢            10                    ,                      000            ⁢                                                  ⁢                          cm              ^              3                        ⁢                          /                        ⁢                          min              /              10                                ,                                    000              ⁢                                                          ⁢                              cm                ^                2                                      =                          1              ⁢                                                          ⁢                              cm                /                min                                                        Moreover, Manz describes in great detail how the various deeper layers of his bio-sand filter could be adjusted in depth and particle size composition in order to control the face velocity at the top of the exposed sand layer. In effect, one of the primary reasons for the large mass of sand and gravel in the deeper layers is to establish and control back-pressure so that the face velocity through the sand bed is kept within the recommended range. In the Manz filter design (AKA, the HydrAid BioSand Water Filter) the exposed surface of the sand is circular and is approximately 12 inches (30.5 cm) in diameter. Using the Manz recommendation the maximum recommended flow rate through the system can be calculated.Exposed Sand Area(A)=Pi*r*r(Pi=3.14 r=radius)A=3.14*15.25*15.25=730.25 cm^2Fmax=maximum recommend flow rateFmax=A*VmaxFmax=730.25 cm^2*1 cm/min=730.25 cm^3/min=730.25 ml/minIt can be seen from the calculation that the flow rate is fairly slow and may not be acceptable to users accustomed to faster flow rates when drawing water for cooking or drinking. Further, the system described by Manz requires a large mass of sand and gravel in order to achieve the desired flow rate.
What is needed then is a water treatment system that is easy to use, does not require electric power or other energy sources, can be used in conjunction with an existing water treatment system or alone, and is easy to maintain. It is desirable for the system to be useful in a variety of applications, such as treating water for consumption in the home, disaster relief and outdoor activities. A water treatment system that is smaller and more portable would also be desirable. In addition, an increased flow rate through the system would enhance ease of use and provide other benefits.